Injection moulding processes for plastics materials have been developed to manufacture items of fixed given dimensions dictated by the size of the mould involved. To injection mould items which have large dimensions, large moulds are required and this entails a substantial cost both in the cost of the mould itself and of the size of the machine required to inject the required amounts of molten plastics material at the required pressures.
Over-moulding processes have been developed whereby features made of a second material are injection moulded directly onto the surface of a component previously injection moulded in a first material but, again, the size of these over-mouldings is constrained by the size of the dies and the plastics injecting equipment used to produce them.
Plastics items of indefinite length can be moulded by extrusion processes. In these processes molten material is extruded from an orifice and is then quickly cooled external of the orifice to form the solid product. Extrusion processes are generally constrained to producing products of constant cross section. Although sophisticated systems have been developed which utilise moving components in or around the orifice whereby a variation in the cross section may be produced in the course of the extrusion, these systems require very high cost dies and control systems.
Extrusion processes, when compared with injection moulding processes, generally require more sophisticated machinery with a higher degree of control over temperatures and production rate. Extrusions also generally require a narrower tolerance of physical properties, and a higher degree of uniformity of the plastic feed material being used, so extrusion processes are therefore generally not suitable for use with recycled plastics materials which can vary in properties quite significantly.
Some virgin plastics materials, such as PET, are not particularly suited for extrusion because their rheological properties during the process are so greatly dependent on the temperature that the necessary temperature control has proven to be almost impossible in an industrial manufacturing environment.
There is a need for a process which provides the abilities of extrusion processes to make products of indefinite length but also provides the simplicity, ruggedness, low cost, simple control, and tolerance to varying feed materials that is provided by injection moulding processes.
An aim of the present invention is to provide a moulding process which addresses these difficulties.